This invention relates to a microwave cooker or oven and, particularly, to a magnetron having a means for reducing the emission or leakage of spurious electromagnetic waves, e.g., the fifth harmonic, included in the oscillated output power.
Microwave cookers use the microwave of 2.45 GHz band for heating and cooking food, and a magnetron which is generally used for this also produces harmonics in addition to the 2.45 GHz fundamental wave. A project of television broadcasting (ABD) in which television programs are directly transmitted to television sets in general households using a broadcast satellite is now under way (and in Japan such has been already practiced), and it is expected to use a 11.7-12.7 GHz band. Therefore, microwave cookers are requested to reduce the leakage of harmonic components, particularly the fifth harmonic component (12.25 GHz), in order to prevent interference to the satellite broadcasting. Some conventional microwave cookers are known to have a microwave absorber made of ferritte or soft ferrite provided at the door. However, such ferrite absorber cannot satisfactorily reduce the harmonic components since the ferrite absorber itself is limited in performance and is very expensive and since the harmonic reduction is not tackled on the microwave sourcing magnetron.
FIG. 1 is a partial cross-sectional view of the conventional magnetron having a means for preventing spurious emission of microwave. In the figure, reference number 1 denotes an anode cylinder, which is provided in its interior room with a plurality of centripetal vanes 2, so that both members in combination constitutes a resonant cavity. A cathode (cathode filament) 10 is located at the center of the anode cylinder 1, and an interaction space is formed between the cathode 10 and the vanes 2. A magnetic pole piece 7 is fixed by soldering or arc welding hermetically to each end of the anode cylinder 1, and it serves to focus or concentrate the magnetic flux produced by an external magnet (not shown) to the interaction space. A stem 11 is to support the cathode 10, and it is fixed hermetically through the magnetic pole piece 7. An output choke 6 is disposed to prevent spurious emission of microwave, and it is welded or soldered to a metallic output sealing member 5 e.g., made of Fe, FeNi alloy or FeNiCo alloy. A ceramic cylinder 9 is secured hermetically to the magnetic pole piece 7 through the output sealing members 5 and 5' which are equipotential with the anode 1. An exhausting tube or pipe 12 made of copper or the like is secured hermetically to the upper end of the ceramic cylinder 9, and it also serves as an output antenna. An antenna lead wire 4 has its one end connected with one of the vanes 2 and another end passing through a hole 13 in the magnetic pole piece 7 and fixed to the exhausting tube 12, namely it is press-welded to the exhausting tube 12 when it is chipped off. A metallic cap 14 is put on the exhausting tube 12 for protecting the press-welded portion of the exhausting tube 12 and antenna lead wire 4. Strap rings 3 are fixed on the upper and lower ends of the vanes 2.
The foregoing structure of magnetron is described, for example, in Japanese Utility Model Laid-open Publication No. 54-125564. However, its output choke alone could not perfectly prevent the leakage of spurious microwave or harmonics. The above-mentioned publication does not clarify the relation between the dimensions of the choke for a specific spurious microwave, e.g., the inner diameter and length of the choke and their effect of leakage prevention. On this account, this prior art magnetron structure has not achieved satisfaction in leakage prevention against spurious microwave or harmonics.